The present invention relates to a heavy duty radial tire, more particularly to a heavy duty radial tire with an aspect ratio of 50% or less in which uneven wear and cracking in groove bottom are suppressed by improving the belt layer.
In heavy duty radial tires used for vehicles such as trucks and buses, as shown in FIG. 8, a belt layer “a” which is disposed radially outward of a carcass is generally composed of three or four belt plies “b” of steel belt cords in which disposed on the radially innermost side is a first belt ply b1 of belt cords arranged at an angle of 60±15° with respect to the tire equator, and subsequently disposed radially outward of the first ply are second and third belt plies or second to fourth belt plies of belt cords arranged at an angle of 10 to 35° with respect to the tire equator. The 2nd and 3rd belt plies b2 and b3 are stacked so that the belt cords in the 2nd ply cross the cords in the 3rd belt ply. By such an arrangement is formed a triangle structure that belt cords cross each other between the 1st and 2nd belt plies b1 and b2 and between the 2nd and 3rd belt plies b2 and b3, whereby the belt rigidity is increased to reinforce a tread portion by a hoop effect.
With increase of expressway and development of high performance vehicles, use of low aspect ratio tires as heavy duty tires is increasing since the aspect ratio of the section height of tire to the section width of tire is low and therefore the steering stability is good.
However, in case of low aspect ratio tires, particularly tires having an aspect ratio of 50% or less, a conventional belt layer cannot exhibit a sufficient binding force since the tread portion is wide and the tread profile is flat. Thus, increase or growth of outer diameter (size growth) in a tread portion, particularly in tread shoulder regions, during running becomes large. As a result, the ground contact pressure increases in the tread shoulder regions to induce uneven wear, and separation at belt edges is easy to occur owing to temperature rise. Further, a stress which acts on the bottom of grooves formed in the tread shoulder portions increases to cause damages such as cracking at the groove bottom.
It is an object of the present invention to provide a low aspect ratio heavy duty radial tire in which the binding force of the belt layer is enhanced to suppress the outer diameter growth in a tread portion, particularly the outer diameter growth in tread shoulder regions.
A further object of the present invention is to provide a low aspect ratio heavy duty radial tire having an aspect ratio of at most 50% capable of preventing occurrence of uneven wear, belt edge separation (cord loosening) and cracking at the bottom of tread grooves.
These and other objects of the present invention will become apparent from the description hereinafter.